ROP of lactones and lactides is an addition polymerization reaction where the terminal end of a polymer acts as a reactive center and cyclic monomers join at that site to increase the length of a larger polymer chain. In the case of a polylactide, an alkoxide of a metal having d orbitals, such as, aluminum or tin, can be used as an initiator of ROP of lactide (see, e.g., Biopolymers from Renewable Resources, Hartmann et al. (eds.), 1998, Spring-Verlag, Berlin, herein incorporated by reference in entirety). Stannous octoate or tin(II) bis-2-ethylhexanoic acid are catalysts that can be used for bulk polymerization of lactide because of solubility in lactide, good reaction rate, high conversion, racemization of less than 1% and synthesis of higher molecular weight polymers (see, e.g., Du et al., Macromolecules (1995) 28(7):2124, herein incorporated by reference in entirety).
Metal-free purely organic catalysts using either tertiary amines or phosphines as nucleophilic transesterification catalysts are known (see, e.g., Nederberg et al., Agnew Chem Int Ed (2001) 40:2712; Nederberg et al., Chem Comm (2001) 2006; and Kamber et al., Chem Rev (2007) 107:5813, each herein incorporated by reference in entirety).
A new class or organocatalysts for ROP is the N-heterocyclic carbenes (NHCs) studied in the polymerization of ε-caprolactone to provide polycaprolactones with controlled molecular weight and low polydispersity (see, e.g., Kamber et al., Macromolecules (2009) 42(5):1634, herein incorporated by reference in entirety).
All of those processes rely on stringent anhydrous conditions and many involve reactions under pressure with solvent.
Polylactide was may be copolymerized with a number of different monomers including other lactone monomers, such as, glycolide and caprolactone. Block copolymers of polycaprolactone and polylactide may be prepared in two possible ways: one, a sequential addition of the required monomer on the living growing chain end of the first monomer; and two, by making a prepolymer with a hydroxyl end group to which the second monomer may be added in the presence of a catalyst (see, e.g., Bero et al., Polym Bull (1993) 21:9; and Veld et al., J. Poly Sci Part A Polym Chem (1997) 35:219, each herein incorporated by reference in entirety).
The reactivity of lactone monomers is much lower than that of lactide monomers which generally excludes using lactones in one-pot processes. There remains a need to generate lactone/lactide block copolymers in a single process format.